In the art and practice of keyless lithographic printing, an integral and important means for controlling the input of ink to the inking rollers of the printing press involves the use of a celled metering roller. Sequential and continuous means are generally supplied to first overfill the well-defined cells in the surface of the metering roller with fresh incoming ink. A scraping or doctor blade is provided to remove virtually all of this excess ink form the metering roller excepting that residing in the cells and the ink-filled cells then transfer a known quantity of ink to an appropriate set of coextensive inking rollers. The inking rolls in turn convey the ink as a more-or-less uniform film to the image areas of the printing plate, thence to the printing blanket, and then to the paper or other substrate being printed in a form corresponding to the image areas of the printing plate. The inking rollers also serve to return that portion of the incoming ink not required to refresh the printing plate image format to a location in the inking system where it is continuously removed and returned to the input portion of the inking system for reuse. Conventional lithographic inking systems do not utilize the removal and reuse components characteristic of keyless inking.
In the practice of lithographic printing it is essential to maintain sufficient water in the non-image areas of the printing plate to assure. that image/non-image differentiation is maintained. This is to assure that ink will transfer only to the image portions of the printing plate format. Many different dampening or water conveying systems have been devised and these systems may be referred to by consulting "An Engineering Analysis of the Lithographic Printing Process" published by J. MacPhee in the Graphic Arts Monthly, November, 1979, pages 666-68, 672-73. Neither the nature of the dampening system nor the nature of the dampening materials that are routinely used in the practice of high speed lithography are expected to place restrictions on utilizing the teachings conveyed in this disclosure.
Reference to R. W. Bassemir or to T. A. Fadner in "Colloids and Surfaces in Reprographic Technology", published by the American Chemical Society in 1982 as ACS Symposium Series 200, will relate that in the art of lithography the inks must be able to assimilate or take up a quantity of water for the lithographic process to have practical operational latitude. Apparently the ink acts as a reservoir for spurious quantities of water that may appear in inked image areas of the plate, since water is continuously being forced onto and into the ink in the pressure areas formed at the nip junction of inking rollers, dampening system rollers, and printing plates of the printing press. Whatever the mechanism might be, all successful lithographic inks when sampled from the inking system rollers are found to contain from about one percent to about as high as 50 percent of water, more or less, within and after a few revolutions to several thousand revolutions after start-up of the printing press. During operation of the press, some of the inking rollers must unavoidably encounter surfaces containing water, such as the printing plate, from which contact a more or less gradual buildup of water in the ink takes place, proceeding eventually back through the inking train, often all the way to the ink reservoir. Consequently, the presence of water in the ink during lithographic printing is a common and expected occurrence.
The first essential property for successful operation of a celled metering roller in keyless lithographic printing is the capability of forming and retaining correctly-dimensioned cells in the surface of the roller during manufacturing. This allows a known amount of ink to be delivered to the inking roller of the press. The technology and art of selecting cell patterns, cell geometry, percentage of non-celled area termed lands or land area, and the like are well-known in the practice of printing with celled roller inkers. It is equally well-known that the three practical means for forming cells in an appropriate metering roller surface are mechanical engraving or knurling or embossing, diamond-stylus engraving or gouging or cutting, and laser engraving or energetic blasting of holes in the roller's surface. One or another of the desired cell patterns and one of these three means for forming the cells are selected depending upon the materials' properties and the materials' requirements for the printing process under consideration.
Cells must be accurately formed in the roller's surface and except when using the more-expensive and less-practiced laser-engraving process with which virtually any practical material can be engraved regardless of hardness, formation of the cells requires that the base roller surface be deformable by, for instance, hardened steel knurling tools. Unhardened steel has been the nearly universal material of choice in prior art celled metering roller technologies. Steel alloys can be selected with appropriate bulk strength, machinability to form the blank roller cylinder, and embossability to form accurate cells.
A second requirement for a celled ink metering roller is resistance to wear erosion of the roller surface and therefore of the cells themselves caused by the scraping blade and by any inking rollers that may be running in physical interference with the metering roller. Generally, this requirement translates into a hardness value of about Rockwell 70 or higher on the C scale. Prior art technologies have utilized chromium plated over copper, nitriding of the steel surface, and flame-sprayed ceramic coatings such as chromium oxide, aluminum oxide or tungsten carbide. Achieving this hardness quality minimizes how often the metering roller must be replaced due to wear in order to maintain consistent day-to-day ink delivery performance of the keyless printing press system.
Previous disclosures have shown that the surface of a metering roller to be used in the lithographic printing process must not only meet the first and second requirements but also must be oleophilic, or oil-loving, and hydrophobic, or water-repelling. This means that when both an oil-based lithographic ink and the dampening water are present at the metering roller's surface, the roller will tend to retain the ink rather than the water on and in its surface and thereby continue to function as an ink metering roller despite the presence of the water. None of the hard materials commonly used in flexographic, letterpress or gravure printing are suitable for use in lithography since they are all hydrophilic.
Although these four just-described propertes are necessary to the formation of an ink metering roller intended for use in keyless lithographic printing, prior art metering roller technologies that meet these criteria may suffer from one or more disadvantages when put to practical use in hard-running printing pressroom environments.
One disadvantage of the prior art technologies utilizing a steel base roller material is their weight, typically from about 150 pounds for a 36-inch long roller to about 400 pounds for a 72 inch-wide printing press. Handling these heavy rollers in the pressroom either to install or replace them requires special fixtures and skills. Light weight metering rollers would represent a distinct practical advantage.
Another disadvantage of all metering roller technologies using a steel base roller is that the steel is subject to oxidative corrosion by diffusion of atmospheric water vapor, dampening water, or any spurious water to the steel surface during manufacture, or shipping, or storage or while in use as a lithographic ink meter roller. Corrosion of the steel surface can totally disbond the coatings that may have been applied during manufacture to render the roller surface hard and wear resistant. As pointed out in British Patent 1,585,413, subsequent use of a coated but corroded roller in the intended operating mode running against a scraping doctor blade may totally remove the coating that was originally intended to impart superior wear resistance. Thus, flame-sprayed ceramic coatings applied over a steel base roller are naturally hydrophilic and porous and therefore require treatment for instance with an adherent water-impermeable organic polymeric material that functions to seal the ceramic layer pores thereby protecting against diffusion of water through the ceramic to the steel core.
Another means for avoiding the effect of corrosion on a steel-based metering roller is to chemically render the steel surface simultaneously hard and corrosion resistant, for instance by nitriding the steel as disclosed by Fadner et al in U.S. Pat. No. 4,537,127 and by Sato et al in U.S. Pat. No. 4,637,310. Chemical conversion of the steel surface to an iron nitride results in a hard surface layer that remains an integral part of the base steel roller and unlike the sharp boundary typical of an applied coating, has a naturally stronger diffuse or gradual boundary leading from the bulk steel to the hard nitrided surface layer. This property together with the higher inherent resistance of nitrided steel to oxidative corrosion appropriately renders the surface of the base steel alloy both wear and corrosion resistant. Rollers based on nitriding technology require prior mechanical engraving of the unhardened steel surface. Both these prior art technologies necessarily result in heavy finished meter rollers. Both also rely on use only of mechanical engraving to form the cells.
Fadner in U.S. Pat. No. 4,601,242 discloses means for rendering the surface of a celled base roller hard and oleophilic and hydrophobic by applying a thin copper coating to the celled base roller followed by a thin porous flame-sprayed ceramic coating such as alumina over the copper. The copper layer serves to protect or seal the steel from spurious corrosion due to the omnipresent water and presents an oleophilic and hydrophobic surface upon which to anchor the oily lithographic ink despite the presence of water once the ink has migrated through the thin, porous ceramic coating. Thus the outermost surface of the roller is celled because of the thinly applied coatings, hard because of the last applied ceramic material and once filled with ink functions as an oleophilic and hydrophobic surface for the subsequent metering of ink on press. This technology is also limited to the art of mechanical engraving to form the cells and by stated example involves use of a heavy steel base roller material. This technology involves application of two distinct material layers and therefore has two interfacial boundaries, steel to copper and copper to ceramic, both of which could fail because of chemical or mechanical stresses imposed during manufacture or during use on a printing press.
In yet another approach, Fadner in 4,567,827 avoids the perceived wear and corrosion disadvantages of uncoated engraved steel rollers by first applying to a suitably engraved base roller a hardenable electroless nickel layer, heating the roller to harden the nickel, then applying a thin copper layer on the nickel to supply the required oleophilic and hydrophobic properties. In this technology any suitable base roller material such as a steel or aluminum alloy may be used Fadner has disclosed that at least some of the copper stays in place on the nickel during doctor blade scraping for up to 40 million printing impressions. It must however eventually wear off of the relatively smooth nickel base coating, exposing the oleophilic but hydrophilic nickel layer, producing roller failure because the roller can no longer pick up ink in the presence of both ink and water. Additionally, this technology also has two interfaces that can potentially fail because of chemical and/or mechanical stresses, namely steel to nickel and nickel to copper.
A particular disadvantage in using a steel base roller to fabricate any celled metering roller for use in lithographic printing press systems is that the required cells cannot be formed by means of diamond-stylus technology. Though embossable, as by hardened mechanical engraving tools, steel alloys are too hard for practical, repetitive cutting by the diamond-stylus technique. Thus, otherwise advantageous and relatively operator-independent, electronically-controlled diamond stylus techniques such as represented by Hell Helioklishograph and American Engraving and Machine Co., which are well-developed for use in cutting cells in softer metals such as copper for manufacture of rotogravure printing cylinders, are precluded form use in manufacture of most prior art ink metering rollers for keyless lithography.
There exists a need for light-weight, easily handled celled ink metering rollers in keyless lithographic printing systems that require a minimum number of failure-prone add-on coatings to render the roller hard, oleophilic and hydrophobic.